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The Cassini spacecraft, which began its tour of the Saturn system just over a month ago, has detected lightning and a new radiation belt at Saturn, and a glow around the planet's largest moon, Titan.

The magnetospheric imaging instrument onboard Cassini recently discovered a new radiation belt just above Saturn's cloud tops, up to the inner edge of the D-ring. Before this discovery, it was not anticipated that such a trapped ion population could be sustained inside the rings.
Credit: NASA/JPL/APL

The spacecraft's radio and plasma wave science instrument
detected radio waves generated by lightning. "We are detecting
the same crackle and pop one hears when listening to an AM
radio broadcast during a thunderstorm," said Dr. Bill Kurth,
deputy principal investigator on the radio and plasma wave
instrument, University of Iowa, Iowa City. "These storms are
dramatically different than those observed 20 years ago."

Cassini finds radio bursts from this lightning are highly
episodic. There are large variations in the occurrence of
lightning from day to day, sometimes with little or no
lightning, suggesting a number of different, possibly short-
lived storms, at mid- to high latitudes. Voyager observed
lightning from an extended storm system at low latitudes, which
lasted for months and appeared highly regular from one day to
the next.

The difference in storm characteristics may be related to very
different shadowing conditions in the 1980s than they are now.
During the Voyager time period when lightning was first
observed, the rings cast a very deep shadow near Saturn's
equator. As a result, the atmosphere in a narrow band was
permanently in shadow -- making it cold -- and located right
next to the hottest in Saturn's atmosphere. Turbulence between
the hot and cold regions could have led to long-lived storms.
However, during Cassini's approach and entry into Saturn's
orbit, it is summer in the southern hemisphere and the ring
shadow is distributed widely over a large portion of the
northern hemisphere. This causes the hottest and coldest
regions to be far apart.

A major finding of the magnetospheric imaging instrument is the
discovery of a new radiation belt just above Saturn's cloud
tops, up to the inner edge of the D-ring. This is the first
time that a new Saturnian radiation belt has been discovered
with remote sensing.

This new radiation belt extends around the planet. It was
detected by the emission of fast neutral atoms created as its
magnetically trapped ions interact with gas clouds located
planetward of the D-ring. With this discovery, the radiation
belts are shown to extend far closer to the planet than
previously known.

"This new radiation belt had eluded detection by any of the
spacecraft that previously visited Saturn. With its discovery
we have seen something that we did not expect, that radiation
belt particles can 'hop' over obstructions like Saturn's rings,
without being absorbed by the rings in the process," said Dr.
Donald G. Mitchell, instrument scientist for the magnetospheric
imaging instrument at the Johns Hopkins University Applied
Physics Laboratory, Laurel, Md.

Saturn's largest moon, Titan, is also shining for attention.
Cassini's visual and infrared mapping spectrometer captured
Titan glowing both day and night, powered by emissions from
methane and carbon monoxide gases in the moon's extensive,
thick atmosphere.

The glow of Titan's extensive atmosphere shines in false colors in this view of Saturn's gas-enshrouded moon acquired by the Cassini spacecraft visual and infrared mapping spectrometer during the July 2, 2004, flyby. This image is a combination of near-infrared colors, each of which probes different phenomena in the moon. Credit: NASA/JPL/University of Arizona

"Not only is Titan putting on a great light show but it is also
teaching us more about its dense atmosphere," said Dr. Kevin
Baines, science team member for the visual and infrared mapping
spectrometer at Jet Propulsion Laboratory, Pasadena, Calif.
"What is amazing is that the size of this glow or emission of
gases is a sixth the diameter of the planet," he added.

The Sun-illuminated fluorescent glow of methane throughout
Titan's upper atmosphere -- revealing the atmosphere's immense
thickness and extending more than 700 kilometers (435 miles)
above the surface, was expected. However, the nighttime glow,
persistently shining over the night side of Titan, initially
surprised scientists.

"These images are as if you were seeing Titan through alien
eyes. Titan glows throughout the near-infrared spectrum. If you
were an alien it would be hard to get a good night's sleep on
Titan because the light would always be on," Baines said.

The Cassini-Huygens mission is a cooperative project of NASA,
the European Space Agency and the Italian Space Agency. JPL
manages the Cassini-Huygens mission for NASA's Science Mission
Directorate, Washington.